(1) Field of the Invention
The present invention relates to a method and apparatus for automatically calibrating the pressure and flow within a ventilator, and in particular to a method and apparatus for automatically calibrating the flow, lung pressure, mouth pressure and reservoir pressure signal transducers within a ventilator responsive to calibration measurements.
(2) Description of the Prior Art
Researchers often evaluate the performance of the lung under various test conditions. One common way to evaluate the lung requires the subject to be anesthetized and ventilated with a respirator. In addition, compounds may be delivered through a special aerosol apparatus which is integrated with the ventilator. During the testing, the animal's airflow in and out of his lung is measured along with one of many possible lung pressures. Which lung pressure to measure is dependent upon the experimental needs of the researcher. Typically, this apparatus is connected to a host PC to perform the data collection.
A ventilator is generally comprised of a source of predetermined air pressure, an inspiration port connectable to the trachea of a test subject, a conduit from the air source to the inspiration port, and means to control the volume of air flowing through the conduit from the air source to the inspiration port. The ventilator also includes an expiration port in communication with the test subject to remove inspired air, and an exhaust port to discharge air or for collection of the expired air for analysis.
The inspiration and expiration ports are normally connected via a Y-tube to the subject's trachea. The exhaust port may be vented to the atmosphere or connected to means to capture expired gases or to maintain a minimum positive pressure on the lung, which is known as positive end-expiratory pressure, or PEEP. The ventilator also optionally includes a nebulizer inline between the ventilator inspiration port and the trachea.
Flow and pressure signals from the subject apparatus, e.g., an enclosure surrounding the subject, are conditioned by a preamplifier prior to reading and analysis of the results by a computer, normally the host PC. The ventilator, preamplifier and a nebulizer controller can be combined with a central processor into a control unit that is in communication with, and receives commands and parameters from the host computer.
Operation of the ventilator is controlled in accordance with a preset program by a processor, which determines the pressure of the air source, the volume of air flowing out of the inspiration port, and opening and closing of inspiration and expiration ports. Control of the air pressure and flow is normally achieved by opening and closing of valves within conduits in the ventilator, with the pressure and flow within the conduits at a given time being measured by inline transducers that transmit electrical signals proportional to pressure or flow to the processor.
In order for the ventilator to function properly, these pressure and flow signals must be precise. Therefore, before operation of the ventilator, it is the practice to calibrate each of the transducers to ensure that the electrical signals accurately reflect the measured pressure or flow. Historically, this calibration has been done manually by applying a predetermined pressure or flow within the relevant conduits and measuring the transducer voltages at given known pressures and flows. This procedure is time consuming and may be inaccurate due to human error.
Therefore, there is a need for a method and apparatus for automatically calibrating a ventilator, and specifically for a method and apparatus for calibrating the pressure and flow transducers signals in a ventilator.